The present application relates to load-current-controlling circuitry, and more particularly, to a novel phase-control circuit for operating a lower-voltage resistive load from a higher-voltage A.C. line, especially where the load is capable, at initiation of operation, of drawing in-rush currents which are much larger than the normal operating current thereof.
It is often desirable to operate a lower-voltage load from a higher-voltage source periodic waveform. Typically, such loads may be utilized for heating or lighting elements, and are fabricated of materials, such as tungsten and the like, having a large temperature coefficient of resistance. Hitherto, use of phase-control circuitry for controlling the magnitude of the load current, by switching the load across the line at a specific time after the start of each line waveform half-cycle and allowing load current conduction for the remainder of that source waveform half-cycle, placed relatively high stress on switching devices in series with the load, due to the relatively high in-rush current drawn by the "cold" load. It is not unusual for starting in-rush currents to exceed the normal operating current by a factor of 10 to 15 times. Thus, the semiconductor switches utilized to control such loads had hitherto been required to safely handle the peak in-rush current, rather than the much smaller operating current. The size and cost of such semiconductor switching devices are considerably greater than for a semiconductor device which is chosen to handle only slightly more than the normal operating current of the load, to provide an adequate safety margin. This problem is of particular importance when non-regenerative switching devices, such as power field-effect transistors (FETs) or insulated-gate transistors (IGTs), are utilized rather than regenerative devices, since the non-regenerative devices can be destructively driven out of the desired saturation operating region or suffer latch-up conditions, if not rated to handle the starting peak current of the load. It is therefore desirable to provide a circuit which will not only limit switching device peak currents with loads of the type having starting in-rush currents potentially an order of magnitude or more greater than the normal operating current, but also to provide a soft-starting circuit which will also control a load parameter, e.g. load power or the like, to afford maximum protection for circuit switching devices rated to obtain a lower-cost load control means.